Monoclonal antibodies are currently being used to diagnose and treat cancer (Mach, J., et al., Current Opinion Immunol. B, 685-693 (1991); Grossbard, M. L., et al., Blood 80 (4):863-878 (1992)). To be useful for therapy, the antibody should recognize an antigen that is present in large amounts on the cancer cells and in negligible amounts in normal cells. Alternatively the antigen can be present in substantial amounts on normal cells, if the normal cells are not components of an essential organ. This approach has been useful in developing new treatments for leukemias and lymphomas. Several differentiation antigens have been identified on lymphomas and leukemias which are good targets for immunotherapy, because they are not present on the stem cells which give rise to differentiated lymphocytes (Grossbard, M. L., et al., Blood 80 (4):863-878 (1992)). Thus, normal lymphocytes that are killed by immunotherapy can be regenerated. Some examples of lymphocyte antigens of this type are CD19, CD22, CD25 and CD30 (Grossbard, M. L., et al., Blood 80 (4):863-878 (1992); Engert, A., et al., Cancer Research 50, 84-88 (1990)). Clearly, it would be very useful to have antibodies that recognize differentiation antigens on solid tumors, but only a small number of these are available. One reason contributing to the paucity of such antibodies is that efforts to identify differentiation antigens on various types of epithelial cells have been relatively modest compared with the intense efforts made to identify differentiation antigens on cells of the hematopoietic system.
Ovarian cancer represents one of the diseases which could be treated by immunotherapy, because the ovaries are always removed during surgery for this disease and reactivity with normal ovarian tissue is not a problem. Several antibodies that recognize differentiation antigens on ovarian cancer cells have been generated. One of these is OC125 that recognizes the CA125 antigen (Bast, R., et al., N. Eng. J. Med. 309, 883-887 (1983)). CA125 is a high molecular weight glycoprotein that is shed by ovarian cancer cells and has been useful in the diagnosis of ovarian cancer. However, antibodies to CA125 are not useful for immunotherapy because the CA125 antigen is shed into the blood stream (Bast, R., et al., N. Eng. J. Med. 309, 883-887 (1983)). Another is MOV18 which recognizes the folate binding protein. This protein is abundant in ovarian cancers as well as in some other tumors. Unfortunately, this protein is also abundantly expressed in kidney (Campbell, I. G., et al., Cancer Res. 51, 5329-5338 (1991)). An antibody we previously isolated and termed MAb K1 reacts with many ovarian cancers and many mesotheliomas. Like OC125, the antibody also reacts with normal mesothelial cells, but it does not react with other cell types except for weak reactivity with some cells in the trachea (Chang, K., et al., Int. J. Cancer 50, 373-381 (1992); Chang, K., et al., Cancer Res. 52, 181-186 (1992), see also U.S. Pat. No. 5,320,956). The antigen recognized by MAb K1 appears to be a differentiation antigen present on mesothelium and is expressed on cancers derived from mesothelium such as epithelioid type mesotheliomas as well as on most ovarian cancers. Thus immunotherapy directed at the CAK1 antigen should take into account the potential risk of damaging normal mesothelial cells and perhaps cells of the trachea (Chang, K., et al., Int. J. Cancer 50, 373-381 (1992); Chang, K., et al., Cancer Res. 52, 181-186 (1992); Chang, K., et al., Int. J. Cancer 51, 548-554 (1992); Chang, K., et al., Am. J. Surg. Pathol. 16, 259-268 (1992)).
Using the ovarian cancer cell line OVCAR-3 as well as HeLa cells, the antigen has been shown to be an approximately 40 kD glycoprotein that is attached to the cell surface by phosphatidylinositol. The protein is released when cells are treated with phosphotidylinositol specific phospholipase C (Chang, K., et al., Cancer Res. 52, 181-186 (1992)). We had previously attempted to clone a cDNA encoding the CAK1 antigen but instead cloned cDNAs encoding two different intracellular proteins which also react with MAb K1 (Chang, K., and Pastan, I., Int. J. Cancer 57, 90-97 (1994)). Neither of these is the cell surface membrane antigen recognized by MAb K1.